On Oct 25, 2013, at 7:49 PM, Allen Wirfs-Brock <al...@wirfs-brock.com> wrote:
> It turns out that even in pseudo code, this is a fairly complicated set of
> runtime validation rules to apply. I'm having a hard time convincing myself
> that the runtime computational and meta data costs of this dynamic validation
> is justified. It costs too much and the actual benefit is pretty small.
>
> ***For that reason, I propose that we drop this runtime validation of object
> literals (and class definition). We would still have the static validation
> and early errors for property definitions that don't have computed keys. But
> anything that makes it past those checks (including all property definitions
> with computed names) are just processed sequentially with no duplicate name
> checking.***
Yep, I fully agree. And I especially appreciate what you say below:
> What about predetermining the "shape" of literal objects that include
> computed property keys?
>
> One of the reason for the dynamic checks was the desire to preserve the
> characteristics that allowed an implementation to look at an object literal
> and statically determine exactly how many own properties the resulting
> objects would have. The hope was that an implementation could still
> determine an intended shape and that the dynamic checks would guarantee that
> the actual constructed objects conform to that predicted shape. If we drop
> the dynamic checks we loose that shape guarantee.
>
> I think this ability to predict an intended shape was probably a committee
> pipe-dream.
Amen.
> Consider this function:
>
> function makeObj(a,b,c,d,e) {
> return {
> get [a] () {},
> get [b] () {},
> set [c] (v) {},
> set [d] (v) {},
> set [e] (v) {}
> }
> }
>
> The object returned by this function might validly have 3, 4, or 5
> properties. The is no clearly intended shape to try to guarantee.
The "shape predictability" concept has never really been defined clearly, and
this is a good demonstration of why it's fishy.
Anyway, agreed 100%. It's worth revisiting at the November f2f.
Dave
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